303 is good to apply on clean vinyl. I would use magic erasers on the vinyl before applying the 303( Aerospace 303 protectant) On the outside of the boat when doing a full detail I use a porter cable and use meguairs cleaner polish on a polishing pad... this will usually get rid of any waterspots, and then go over that with a wax of choice.. Klasse AIO is great. NXT Window cleaner I find to be my favorite on windows. Then during wipe down if you fill a sprayer up with vinegar it should help with water spots.

If you want the BEST one step process got to www.boatbling.net and get HOT SAUCE. I have tried the rest and this is the best water spot remover/ wax I have used. Yes it cost more at about $16-18 a bottle with shipping. Let me guess you spent over $20K for your boat, spend some coin on keeping it looking good. Also the Vinyl Sauce is very good as well. I also like the 3M vinyl restorer if you have older vinyl that needs to be cleaned up and taken care of. As for wax and polish I really like the driven product line. www.driventoperfection.com it does a great job.